Electron discharge device



1951 w. A. DICKINSON ETAL 2,971,108

ELECTRON DISCHARGE DEVICE Filed Sept. 26, 1958 INVENTO W/LL/AM ,4.fiICIM9ON 650,755 5 H455LEE A'ITORNEY United SttCS Patent I ELECTRONDISCHARGE DEVICE William A. Dickinson, Seneca Falls, and George E.

Hassler, Auburn, N.Y., assignors, by mesne assignments, to SylvaniaElectric Products Inc., Wilmington, Del., a corporation of DelawareFiled Sept. 26, 1958, Ser. No. 763,593

2 Claims. (Cl. 31382) This invention relates to electron dischargedevices and more particularly to cathode ray tubes having relativelyhigh transconductance.

One type of cathode ray tube adapted to be employed in electricalapparatus such as television receivers utilizes a low G electron gun forachieving relatively high transconductance. This gun has applied to thesecond grid a voltage, e.g. 100 volts positive, which is low compared tothe conventional tubes utilizing approximately 300 volts positive on thesecond grid. Since the transconductance of this type of cathode ray tubeincreases with a decrease in the second grid voltage, it is desirable tolower this voltage as much as practicable. One of the advantages of ahigh transconductance tube resides in its ability to operatesatisfactorily with a relatively small video drive signal. This isextremely desirable since the present trend in television circuit designis to utilize either lower output video amplifiers or to delete theamplifier and couple the cathode ray tube control electrode, e.g., thecathode or first grid, directly to the second detector.

Previous attempts to fabricate a low G tube have been unsuccessful sincethe electron gun design was so critical that it could not beeconomically made, or since the spacings between electrodes and theelectrode aperture sizes selected were such that a large percentage ofrejects occurred from shorts due to close spacings, from low beamcurrent due to wide spacings, and from poor electron beam focusing.

Accordingly, it is an object of the invention to reduce theaforementioned disadvantages and to achieve relatively hightransconductance in a cathode ray tube capable of economic fabrication.

A further object is to reduce rejects normally occurring from thecritical construction of a low G cathode ray tube.

The foregoing objects are achieved in one aspect of the invention by theprovision of a cathode ray tube having an electron gun assemblyincluding a cathode, a first grid and a second grid. The first grid is acylinder having an end wall formed to provide an aperture aligned withthe cathode. The end wall surrounding the aperture has a given thicknesswith the cathode to first grid spacing being from 1.25 to 3.0 times thisgiven thickness. The distance between the first and second grids is from3 to 7 times the given thickness. With this unique construction, thetube operates satisfactorily with a voltage on the second grid at 50volts positive or lower.

For a better understanding of the invention, reference is made to thefollowing description taken in conjunction with the accompanyingdrawings in which:

2,971,108 Patented Feb. 7, 1961 C we for use as a television picturetube is shown for purposes of illustration. The tube comprises anenvelope 11 having an electron. gun 13 mounted therein spaced from thefluorescent screen 15. The electron gun provides the source,acceleration, control and focusing of the electron beam 17, which isdeflected by coils 19 to strike the screen 15 and provide the imagedisplay raster forthe tube.

The electron gun 13, Fig. 2, is shown comprising'a cathode assembly 21including a cylindrical sleeve 23 having electron emissive material 25deposited on the end thereof. A heater 27 provides the heat necessary tocause electrons to be emitted from material 25. Spaced from the cathodealong the path of electron travel is a first grid 29, a second grid 31,an electron accelerating anode 33, and a uni-potential electrostaticfocus assembly 35.

The electron beam 17 is focused by assembly 35 to provide a spot ofminimum diameter at the impinging position on screen 15. This assemblycomprises the spaced lens cups 37 and 41 having apertures 40 and 42formed therein surrounded at their aperture regions by a lens ring 3 9.Assembly 35 functions in such a manner that the divergent electron beam17 arriving from the cathode 23 is made convergent from the focusingassembly 35 to the fluorescent screen 15.

Referring to Fig. 3, the first grid 29 is formed as a cylinder having anaperture 43 aligned with the cathode material 25. The end wall 45 of thefirst grid is coined or operated upon by other means to reduce thethickness of the wall adjacent aperture 43. The coining operation causesa recess to be formed in wall 45 facing grid 31. The second grid 31,which also has a cylindrical form, is provided with an aperture 47aligned with the first grid aperture.

It has been found that the cathode to first grid and first grid tosecond grid spacings, and particularly the thickness of wall 45 adjacentaperture 43, is instrumental in fabricating a low G hightransconductance cathode ray tube. The dimensions and spacings shown inFig. 3 illustrate a construction which provides excellent video driverequirements since it allows the application of a voltage on the secondgrid 31 as low as 50 volts. Grid 29 has an outside diameter of .520 inchwith a length of .300 inch. The aperture 43 has a diameter of .036 inchand end wall 45 is coined down from .007 inch thickness to .0025 inchadjacent aperture 43. The second grid also has an outside diameter of.520 inch and has a length of .225 inch. The wall thickness is .0090inch while aperture 45 has a diameter of .036 inch.

The electron emissive material 25 at the end of cathode 21 is spacedfrom the internal edge of wall 45 a distance of .0045 inch. Second grid31 is spaced from first grid 29 a distance of .0115 inch measured fromthe surface of wall 45 surrounding the aperture and facing grid 31.

It has been found that the spacings and dimensions of anode 33 and thefocusing assembly electrodes is not critical in constructing a low Gtube. However, these electrodes should be assembled to provide adequatefocusing at screen 15. If desired, other forms of electrostatic ormagnetic focusing may be employed in lieu of assembly 35.

The structural dimensions shown in Fig. 3 are for purposes ofillustration. However, it is preferable to achieve a first grid wallthickness adjoining aperture 43 as small as practicable. Excellentresults have been obtained using a thickness of from .0020 inch to .0030inch. Once this given thickness is acquired, the spatial distancebetween cathode 21 and first grid 29 is prefer= ably from 1.25 to 3.0times this given thickness and the distance between grids 29 and 31 ispreferably from 3 to 7 times this given thickness to obtain reasonabletube characteristics.

A cathode ray tube of the type described herein has relatively hightransconductance and therefore requires a small amount of video drive.When the tube is cathode driven as shown in Fig. 2, and with 5.0 voltspositive on second grid 31, a video drive on cathode 21 need only befrom approximately 35 volts positive to zero bias to traverse fromapproximately 1000 micro-amperes beam current to beam 'cut-ofi. Thesevalues exemplify excellent drive characteristics and allow utilizationof low voltage sources.

The anode 33 and lens cups 37 and 41 have an accelerating potential of18 kv. imposed thereon while lens ring39 is operated at 300 volts.Conventionally, the 16 kv. electrodes are connected to the screen 35 ofthe tube to provide a uni-potential deflection region for beam 17.

Although several embodiments of the invention have been shown anddescribed, it will be apparent to those skilled in the art that variouschanges and modifications may be made therein without departing from thescope of the invention as defined by the appended claims.

What is claimed is:

1. An electron discharge device employing an electron beam comprising animage screen, and an electron gun formed to provide the source,acceleration, control and focusing of the electron beam spaced from saidscreen, said electron gun including a cathode, a first grid and a secondgrid arranged in the direction of electron travel toward said screen,said first grid comprising a cylinder having an end wall formed toprovide an aperture aligned with said cathode, the surface of said endWall surrounding the aperture having a given thickness of from .0020 to.0030 inch, the spatial distance between the cathode and first gridbeing from 1.25 to 3.0 times said given thickness and between said firstand second grids being from 3.0. to 7.0 times said given thickness.

2. An electron discharge device employing an electron beam comprising animage screen, and an electron gun formed to provide the source,acceleration, control and focusing of the electron beam spaced from saidscreen, said electron gun including a cathode, first grid, second grid,anode and an electrostatic focusing electrode assembly arranged in thedirection of electron travel toward said screen, said first gridcomprising a cylinder having an end wall formed to provide an aperturealigned with said cathode, the surface of said end wall surrounding theaperture having a given thickness of from .0020

to .0030 inch, the spatial distance between the cathode and first gridbeing from 1.25 to 3.0 times said given thickness and between said firstand second grids being from 3.0 to 7.0 times said given thickness.

References Cited in the file of this patent UNITED STATES PATENTS2,173,498 Schlesinger Sept. 19, 1939 2,732,511 Dichter Jan. 24, 19562,806,163 Benway Sept. 10, 1957 2,840,739 Lesovicz June 24, 19582,888,588 Dichter May 26, 1959

